Add environment checklist to all notebooks

[bsipocz]

Notebooks have various dependency requirements, some use numpy+scipy only, others require astroML, astropy, or pymc3.

When we launch them in colab some of these dependencies are missing, so adding a first cell that runs a script that checks on dependencies might be a better way than to add manual !pip installs to the notebooks for e.g. colab.

This would also be helpful for those who run the notebooks locally.